Who knows if this is the truth about the crusades, but it is an interesting take:

Christians in the eleventh century were not paranoid fanatics. Muslims really were gunning for them. While Muslims can be peaceful, Islam was born in war and grew the same way. From the time of Mohammed, the means of Muslim expansion was always the sword. Muslim thought divides the world into two spheres, the Abode of Islam and the Abode of War. Christianity